The present invention relates to a line card with a modem interface.
Digitized data, such as data produced by a computer, can be transmitted over the plain-old telephone service (POTS) voice-band phone network using a modem. Conventional modems use POTS compatible electrical signaling to establishes a voice-band communication channel through a POTS central office switching system to a destination modem. The originating and destination modems exchange digital data by encoding the data as voice-band signals which are transported end-to-end across the telephone network.
Various transmission standards exist to provide POTS-compatible voice-band encoding of digitized data. For example, modems implementing the International Telecommunications Union (ITU) V.34 standard can support 28.8 Kilobit per second (Kbps) voice-band data transmission and modems supporting the K56flex protocol can support transmission at rates of up to 56 Kbps. The maximum data communication rate between the originating and destination modems is determined, among other things, by the supported modem protocol and the end-to-end electrical interference on the communications channel between the modems.
POTS telephone signals, including those exchanged by POTS voice-band compatible modems, typically originate at POTS customer premises equipment (CPE) and are transmitted over a twisted-pair wire loop terminating at a central office POTS line card. A POTS compatible line card provides signal transmission functions such as ringing voltage pulses, CPE power, dual-tone multi-frequency (DTMF) tone detection, and pulse-dialing detection. Conventional POTS line cards typically interface to a central office POTS switching system (a "POTS switch"). The POTS switch establishes a voice-band data connection by routing a POTS call from the originating CPE to terminating CPE. Typically, POTS voice-band signals are converted to a 64 Kbps pulse-code modulated (PCM) data stream allowing voice-band frequencies of up to 3.1 kilohertz to be routed through the POTS network. As with other POTS-compatible services, and regardless of the modem's supported data rate, the voice-band frequencies produced by a modem are typically encoded as a 64 Kbps PCM data stream for routing through the telephone network.
Routing of modem-originated data through a POTS switch requires POTS switch resources to be dedicated to the voice-band channel between the originating and destination modems. For example, a 64 Kbps time-division multiplexed interoffice trunks may be used to provide the voice-band channel.